I’m not quite sure what the question is but I will try to answer it--somewhat. Edwin Christy was the best thing that ever happened to the young struggling Foster. Foster was able to sell most of his songs directly to Christy, who performed them, popularized them, (and published them). Some of the letters between the two men are available and contain interesting commentary. As for the 3 ½ minute song time I’m sure that this was largely a Foster thing because just about every suggestion Foster made in his letters Christy accepted. Foster and Christy worked together from 1847 until Christy retired in 1854 (a good story remains unwritten about Christy’s sudden death). George Harrington (Christy) took over the business. Foster still had a good 10 years of music left in him and George happily accepted any of Foster’s songs.As for the format of minstrelsy, I suggest that Foster didn’t really care that much about interlocutors and end-men. He was a songwriter first and foremost, and he felt that 3 ½ minutes was about right. So minstrelsy made his music fit.